Many industrially successful processes for operating fluidized bed reactors used for the catalytic polymerization of olefins take advantage of "condensing mode" and "super condensing mode" techniques. These techniques are described in detail in U.S. Pat. Nos. 4,543,399; 4,588,790; and 5,352,749. They entail the cooling of gases and/or fluids in the recycle line of a fluidized bed reactor to form a gas and a liquid phase. The two phase mixture is then directed into the fluidized bed as a fluidizing medium, the liquid portion of which vaporizes when it is exposed to the heat of the reactor. The objective is to take advantage of the cooling effect brought about by the vaporization, i.e., by bringing the temperature of the fluidized bed down to a point where degradation of the polymer and the catalyst are avoided and agglomeration of the polymer and chunking are prevented. The liquid phase is provided by a portion of the recycle gases, which includes monomers and low boiling liquid hydrocarbons, inert to the reaction conditions needed for polymerization, and condensation from which the techniques get their names. The amount of liquid hydrocarbons can be as much as 50 percent by weight of the total weight of the fluidizing medium used in the reactor. A commonly used liquid hydrocarbon is isopentane, which boils at about 27 degrees C., and consequently becomes a vapor in the recycle line in view of the heat present in the recycle gases. The recycle gases leave the reactor, are cooled, and then condensed to the extent that a vapor phase and liquid phase are formed. The velocity of the recycled gas/liquid mixture should be sufficient to support the fluidized bed, but slow enough to avoid excessive entrainment of fines. The cooling capacity should be sufficient to improve the production rate in terms of space/time/yield. Present technology accomplishes both.
The negatives reside in the limit placed upon the production rate, i.e., space/time/yield, which is influenced by the maximum rate at which heat can be removed from the reactor; the introduction of hydrocarbons other than monomers, which, although considered inert, can affect the polymer morphology and the static, chunking, and sheeting, which plague many polymerization runs; flooding and frothing at the bottom of the reactor due to too much liquid; and the requirement for additional cooling and condensing equipment for converting the recycle stream to the gas/liquid mixture.